1. Field of the Invention
This invention relates to pressure-sensitive recording sheets and more particularly to pressure-sensitive developing sheets which can provide recorded color images having a high color intensity and excellent light fastness.
2. Prior Art
Generally, pressure-sensitive recording sheets are composed of a top sheet coated on its backside with microcapsules containing electron-donating colorless or light-colored color-forming dyes dissolved in an organic solvent (capsule oil) and an under sheet having, on its front side, a developing agent layer containing electron-accepting developing agents. These two sheets are laid so that the two coated surfaces face each other and arranged so that printed records are produced in such a way that when pressure is applied with a ball-point pen or typewriter, the capsules in the pressed area are ruptured and the emerged capsule oil containing color-forming dyes is transferred to the developing layer and brings about a color-forming reaction. Moreover, a plurality of copies can be obtained by using intermediate sheets wherein each of which has a developing layer on its front side and microcapsules on its back side, sandwiched between the top and under sheets. Accordingly, the developing sheets of this invention include intermediate sheets in addition to the above-mentioned under sheet.
The developing agents which are conventionally known include inorganic solid acids such as activated clay (for example, Japanese Patent Publication No. 7622/1966), attapulgite (for example, U.S. Pat. No. 2,712,507), substituted phenols and diphenols (for example, Japanese Patent Publication No. 9309/1965), p-substituted phenol/formaldehyde polymers (for example, Japanese Patent Publication 20144/1967), aromatic carboxylic acid metal salts (for example, Japanese Patent Publication No. 10856/1974), and 2,2'-bisphenol sulfone compounds (for example, Japanese Patent Laid-Open No. 106313/1979).
Of these developing agents, activated clay is prepared by treating Japanese acid clay or like clay with a mineral acid to elute iron or other basic components and increase the specific surface area. Activated clay having a specific surface area of not less than 200 m.sup.2 /g is thought to have a particularly excellent developing effect. Such activated clay is advantageous in that it is very inexpensive as compared with the cost of the other above described organic developing agents.
With respect to the color of the developed color image of a pressure-sensitive recording sheet, blue color-forming recording sheets prepared by using, as a developing agent, Crystal Violet Lactone (CVL), Benzoyl Leuco Methylene Blue (BLMB) or the like were initially used. However, as demands of the market for black color-forming recording sheets grew greater, a method comprising mixing at least two dyes having different developed colors has been adopted in order to produce black color recorded images, as disclosed, for example, in Japanese Patent Publication Nos. 4698/1970 and 4614/1971. For example, black color-forming dyes can be obtained by combining dyes having developed colors which are in a relationship of complementary colors or which constitute primary colors, such as blue/yellowish orange, blue/yellow/yellowish orange/red and blue/green/red.
This system is chiefly applied to pressure-sensitive. manifold sheets in which an inorganic solid acid such as activated clay or attapulgite is used. However, because the dyes have different rates of color formation and different fastness to light, temperature or moisture, they have a drawback that the developed color changes with the lapse of time when the images are further exposed to sunlight or ultraviolet rays during the period from the initial color formation to the final color formation or when they are stored for a long time. Furthermore, the following problems are frequently encountered: the color mixing and matching requires much time; because many kinds of dyes are used in quantity, the cost becomes expensive and the dissolution of dyes in solvent (capsule oil) is difficult.
In order to eliminate these drawbacks, fluoran dyes which can form a black color by themselves have been developed.
Exemplary of these dyes are 3-diethylamino-6-methyl-7-anilinofluoran, 3-(N-cyclohexyl-N-methyl-amino)-6-methyl-7-anilonofluoran, 3-diethylamino-6-methyl-7-(o,p-dimethylanilino)fluoran and 3-(N-ethyl-p-toluidino)-6-methyl-7-anilinofluoran. These dyes have an advantage that when an organic developing agent such as p-substituted phenol/formaldehyde polymer, aromatic carboxylic acid metal salt or 2,2'-bisphenol sulfone compound metal salt is used as a developing agent, the above dyes alone or in combination with a small amount of blue, red or like color dyes acting as complementary color dye can produce black developed color images whose color changes little with the lapse of time.
On the other hand, these dyes have disadvantages in that when they are used in combination with an inorganic developing agent such activated clay, the developed image shows reddish black or greenish black color, and the images when exposed to sunlight or ultraviolet rays, discolor or fade to reddish brown, so that developed color images with a stable color cannot be obtained.
Accordingly, although activated clay is inexpensive and has a superior developing performance as compared with organic developing agents, there has been no choice but to adopt the above-described mixed dye system in order to obtain a black color with a clay type developer.